Communication networks use Multiple Input Multiple Output (MIMO) technology to achieve high data rates. MIMO systems use more than one transmit antenna to send a signal on the same frequency to more than one receive antenna. Whereas traditional cellular networks generally provide the best service under line-of-sight conditions, MIMO thrives under rich scattering conditions where signals bounce around the environment. Under rich scattering conditions, signals from different transmit antennas take multiple paths to reach the user equipment at different times. A MIMO system allows to multiply the capacity of a radio link using multiple transmit and receive antennas to exploit multipath propagation.
A MIMO system can employ spatial multiplexing. In spatial multiplexing, a high-rate signal is split into multiple lower-rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receive antenna array with sufficiently different spatial signatures and the receiver has accurate channel state information CSI it can separate these streams into parallel channels. Spatial multiplexing is a powerful technique for increasing channel capacity at higher signal-to-noise ratios SNR. The maximum number of spatial streams is limited by the lesser of the number of antennas at the transmitter or receiver. Spatial multiplexing can also be used for a simultaneous transmission to multiple receivers, known also as space-division multiple access or multi-user MIMO.
Spatial multiplexing works by creating separate data streams on multiple antennas. In conventional spatial multiplexing a baseband apparatus can divide a received high-rate data signal to be sent to a given user equipment on a given subchannel into multiple lower-rate data streams, called MIMO layers L. The number of MIMO layers is the same as the rank of the transmission. In the simplest case of spatial multiplexing, a rank two spatial multiplexing transmission on a 2×2 MIMO antenna configuration does transmit one layer L from each transmission antenna Tx as illustrated in FIG. 1. In this case, the paths 1-1 and 1-2 in FIG. 1 represent layer L1, while paths 2-1 and 2-2 represent layer L2. Each layer L reaches each receiving antenna RX along a different path. The user equipment UE then reconstructs the layers L using information from both antennas.
Multiple layer transmissions either arrive from higher level processes in one or more codewords as illustrated in FIG. 2. In a conventional baseband apparatus, each codeword can then be mapped onto one or more layers L by a mapping unit.
However, conventional MIMO systems make not full use of the flexibility offered by spatial multiplexing providing a limited data transmission capacity.
Accordingly, there is the need to provide a MIMO baseband apparatus providing data transmission capacity using improved spatial multiplexing.